diff --git a/include/apple2.hires.h b/include/apple2.hires.h
index 5a28188..2d3af7a 100644
--- a/include/apple2.hires.h
+++ b/include/apple2.hires.h
@@ -4,6 +4,8 @@
 #include "apple2.h"
 #include "vm_bits.h"
 
+extern int apple2_hires_update(size_t, int);
+extern int apple2_hires_updated(size_t);
 extern void apple2_hires_draw(apple2 *, int);
 extern void apple2_hires_dump(apple2 *, FILE *);
 
diff --git a/src/apple2.dbuf.c b/src/apple2.dbuf.c
index 44fb7e2..d0d5a13 100644
--- a/src/apple2.dbuf.c
+++ b/src/apple2.dbuf.c
@@ -10,6 +10,7 @@
 
 #include "apple2.dbuf.h"
 #include "apple2.draw.h"
+#include "apple2.hires.h"
 #include "vm_reflect.h"
 
 static size_t switch_reads[] = {
@@ -83,20 +84,24 @@ SEGMENT_READER(apple2_dbuf_read)
 SEGMENT_WRITER(apple2_dbuf_write)
 {
     apple2 *mach = (apple2 *)_mach;
+    bool is_lores = (addr >= 0x400 && addr < 0x800);
+    bool is_hires =
+        (addr >= 0x2000 && addr < 0x4000) &&
+        (mach->memory_mode & MEMORY_HIRES);
+    bool is_page2 = (mach->memory_mode & MEMORY_PAGE2);
 
     if (mach->memory_mode & MEMORY_80STORE) {
-        if (addr >= 0x400 && addr < 0x800 && 
-            (mach->memory_mode & MEMORY_PAGE2)
-           ) {
+        if (is_lores && is_page2) {
             segment = mach->aux;
-        } else if (addr >= 0x2000 && addr < 0x4000 &&
-                   (mach->memory_mode & MEMORY_PAGE2) &&
-                   (mach->memory_mode & MEMORY_HIRES)
-                  ) {
+        } else if (is_hires && is_page2) {
             segment = mach->aux;
         }
     }
 
+    if (is_hires) {
+        apple2_hires_update(addr, 1);
+    }
+
     // Again, segment is allowed to be that which was passed in if
     // 80STORE is low. 
     segment->memory[addr] = value;
diff --git a/src/apple2.hires.c b/src/apple2.hires.c
index b415b80..93e7a7f 100644
--- a/src/apple2.hires.c
+++ b/src/apple2.hires.c
@@ -62,6 +62,12 @@ static int addresses[] = {
     0x23D0, 0x27D0, 0x2BD0, 0x2FD0, 0x33D0, 0x37D0, 0x3BD0, 0x3FD0, // 184-191
 };
 
+/*
+ * This table indicates whether an address needs to be updated
+ * (updated[i] == 1) or not (updated[i] == 0).
+ */
+static int updated[0x2000];
+
 #if 0
 /*
  * This table maps an address (across the entire hires graphics buffer
@@ -619,6 +625,30 @@ static vm_color colors[] = {
     { 0xff, 0xff, 0xff, 0x00 },     // white
 };
 
+/*
+ * Mark the given address as one that requires an update on the screen.
+ */
+int
+apple2_hires_update(size_t addr, int needed)
+{
+    if (addr < 0x2000 || addr >= 0x4000) {
+        return ERR_INVALID;
+    }
+
+    updated[addr - 0x2000] = needed;
+    return OK;
+}
+
+int
+apple2_hires_updated(size_t addr)
+{
+    if (addr < 0x2000 || addr >= 0x4000) {
+        return 0;
+    }
+
+    return updated[addr - 0x2000];
+}
+
 /*
  * Draw a single row of hires graphics.
  */
@@ -637,11 +667,19 @@ apple2_hires_draw(apple2 *mach, int row)
 
     for (int i = 0; i < 40; i++) {
         vm_8bit byte = mos6502_get(mach->cpu, addr + i);
+        vm_8bit is_updated = apple2_hires_updated(addr + i);
 
         for (int pos = 0; pos < 7; pos++) {
-            dots[(i * 7) + pos] = 
+            vm_8bit dotval =
+                (is_updated ? 4 : 0) |
                 ((byte & 0x80) ? 2 : 0) |
                 ((byte >> pos) & 1);
+
+            dots[(i * 7) + pos] = dotval;
+        }
+
+        if (is_updated) {
+            apple2_hires_update(addr + i, 0);
         }
     }
 
@@ -650,6 +688,10 @@ apple2_hires_draw(apple2 *mach, int row)
             curr = 0;
 
     for (int i = 0; i < 279; i++) {
+        if (dots[i] & 4) {
+            continue;
+        }
+
         curr = dots[i] & 1;
         next = dots[i+1] & 1;